As a scale inhibitor of this type, there is known a polyphosphoric acid-based scale inhibitor comprised of granular materials containing a plurality of polyphosphate salts (for instance, Patent Literature 1 or 2). PTL 1 or 2 has an object to improve the persistence of the effect of inhibiting occurrence of scale by further mixing a component to be coexistent with polyphosphate salts.
For instance, in the technique described in PTL 1, phosphonic acid, phosphinic acid, polycarboxylic acid and the like are mixed. In addition, in the technique described in PTL 2, alkali metal oxide, which is generally used in a polyphosphoric acid-based scale inhibitor, is mixed with alumina, silica.